This invention relates generally to a communications device and in particular to direct digitally tunable microwave oscillators and filters that enable a low-cost high-speed digital transceiver in the millimeter wave frequency range.
Telecommunication deregulation and the growth of the Internet are causing a large demand for broadband communications network access to homes and offices at increasing bit rates. For Internet access, a Digital Subscriber Loop (DSL) service and similar techniques are becoming popular. However, the speed of the DSL service is limited by the fact that the data travels over long telephone twisted copper wire pairs. Cable modems are also becoming prevalent methods of Internet and telephony access. However, the return channel for a cable modem is quite limited in speed by the nature of cable distribution technology. Fiber optics offers higher access speeds than the other systems and services, but fiber does not currently reach most homes and offices and installation of new fiber is very expensive.
To overcome these limitations with current systems, service providers and users are considering using digital wireless communications as a possible alternative access system. Various radio frequency bands are in fact currently allocated to applications that include broadband wireless access, however, most of the available bandwidth is in the millimeter wave range, ranging roughly from 15 to 66 GHz. Among these bands, the USA Local Multipoint Distributed Services (LMDS) bands in the 28 to 30 GHz range are a good example of a licensed band intended for telecom access applications. Since LMDS is a licensed band, the license owner has the exclusive rights to use the band for such services, thus ensuring interference-free operation.
LMDS services may use point to multipoint communication networks. The LMDS service provider typically maintains base station antennas on tall structures to maintain line of sight to a large number of user-buildings in a sector. Each subscribed user gets a small transceiver installed outdoors. While the base station cost is divided among many users, the subscriber transceiver serves only one user or at the most few users in a shared building. Thus, the cost of the transceiver must be kept low for the LMDS service to be economical. The option of reducing the cost of the transceiver by using a lower frequency is not always practical since it may be precluded by spectrum availability limitations so that millimeter wave transceivers are desirable, but too expensive for many applications. To make millimeter wave communications cost effective, a drastic cost reduction of the millimeter wave transceivers is required.
A typical millimeter wave transceiver includes a frequency synthesizer that generates the final millimeter wave frequency with an offset of a few GHz depending on the particular radio application. The synthesizer is required for accurately setting the exact transmit and receive frequencies of the communications. An undesirable by-product of frequency synthesis is phase noise that must be kept within acceptable performance levels for a particular radio link. Furthermore, a synthesizer may require a large tuning range, usually a few hundred MHz, to allow it to tune to alternate channels. A common way of implementing a synthesizer is to use a base phase locked loop at a lower frequency, such as around 2 GHz, and a chain of frequency multipliers. For example, a 28 GHz signal may be synthesized from a 1.75 GHz signal multiplied by 16 (i.e., 2×2×2×2) which may be implemented using a chain of four frequency doublers.
There are several limitations to the above typical frequency synthesizer. The cost of the synthesizer is significant because the frequency doublers add complexity and cost. Furthermore, the synthesizer is very sensitive to noise. In particular, the phase locked-loop in the synthesizer has a voltage controlled oscillator (VCO). The tuning voltage of that VCO causes the final frequency of the VCO to vary by a few MHz so that one millivolt of noise in the phase locked loop control voltage may cause a frequency deviation of 500 kHz. This frequency deviation may cause temporary loss of frequency lock and large bit errors in the communications data stream which are both undesirable.
Apart from the frequency synthesizer, the millimeter-wave filters in a transceiver also increase the cost of the transceiver, especially in frequency division duplex (FDD) transceivers in which a diplexer is used. The above is also true for a time division duplex (TDD) receiver which also needs filters. Thus, it is desirable to provide a direct digitally tunable oscillator and filter that overcomes the above limitations and problems with typical oscillators and filters that may be used in millimeter wave transceiver devices and systems and it is to this end that the present invention is directed.